The United States has always entered wars by protecting its national interests, whether that be another country attacking us or a threat to our nation. Idealistically the United States may be against an immoral practice another country is doing, but will not start a war unless somehow the United States is threatened or attacked. Our country might give aid to the suffering country or put sanctions against the opposing country that is causing the problems. Only by looking at our nations wars in the past, for example the Spanish-American War, World War I, and World War II we can see our nation will only go to war if our nation is attacked or there is a serious threat to our nation affecting our people or economic situations.
Take for instance World War II. In the 1930's Japan was trying to gain an empire in Asia by attacking areas such as Manchuria, French Indochina and many parts of Northern China. The Japanese militarists slaughtered about 5,890,000 Chinese during this time in World War II. They were unrighteous by going area-to-area raping thousands of women and children. At that time, the United States did not want to enter a war with Japan. Instead of entering a war with Japan pre WWII, the United States put oil sanctions against Japan to put a damper on the Japanese from doing repugnant acts to the Chinese. Incidentally, as the Japanese oil supply diminished this left no option to the Japanese but to attack the United States to gain access to our oil. Thus, because the Japanese attacked us at Pearl Harbor destroying our nations navy, the United States had to enter WWII.
To protect our national interests we allied up with England and Russia after entering a war with Japan. England and Russia during the time that Japan attacked the United States were in a war with German and Italy. England was a good trading partner to the United States. The U.S had a lot of business relations with England and with the threat of Germany almost overtaking there country, we would lose relations and the backing of them if we needed it.